Investigating the potential anticancer activities of antibiotics as topoisomerase II inhibitors and DNA intercalators: in vitro, molecular docking, molecular dynamics, and SAR studies

Abstract Topoisomerase II (TOP-2) is a promising molecular target for cancer therapy. Numerous antibiotics could interact with biologically relevant macromolecules and provoke antitumor potential. Herein, molecular docking studies were used to investigate the binding interactions of 138 antibiotics against the human topoisomerase II-DNA complex. Followed by the MD simulations for 200 ns and MM-GBSA calculations. On the other hand, the antitumor activities of the most promising candidates were investigated against three cancer cell lines using doxorubicin (DOX) as a reference drug. Notably, spiramycin (SP) and clarithromycin (CL) showed promising anticancer potentials on the MCF-7 cell line. Moreover, azithromycin (AZ) and CL exhibited good anticancer potentials against the HCT-116 cell line. Finally, the TOP-2 enzyme inhibition assay was carried out to confirm the proposed rationale. Briefly, potent TOP-2 inhibitory potentials were recorded for erythromycin (ER) and roxithromycin (RO). Additionally, a SAR study opened eyes to promising anticancer pharmacophores encountered by these antibiotics. Highlights Molecular docking studies of 139 antibiotics against the topoisomerase II-DNA complex. SP, RO, AZ, CL, and ER were the most promising and commercially available candidates. Molecular dynamics simulations for 200 ns for the most promising five complexes. MM-GBSA calculations for the frontier five complexes. SP and CL showed promising anticancer potentials on the MCF-7 cell line, besides, AZ and CL exhibited good anticancer potentials against the HCT-116 cell line. Potent TOP-2 inhibitory potentials were recorded for ER and RO.


Introduction
Numerous patients worldwide suffer from variable types of cancer and the prevalence of cancer is increasing 1 . For instance, according to GLOBOCAN 2020, the global number of cancer cases was 19 292 789 while death cases attributed to cancer were 9 958 133 2,3 . Numerous factors contribute to this, such as increased pollution and exposure to ultraviolet radiation 4 . Additionally, change in population age structure and the rise of bad lifestyle habits such as smoking, alcohol consumption, being overweight, obesity, physical inactivity, and malnutrition all contribute to the increased global prevalence of cancer 5,6 . Therefore, the development of cancer therapy is an ongoing research quest. In the past, cancer therapy relied on surgeries and radiotherapy. Currently, molecular cancer cell targets are the main approach for the development of an anticancer agent 7,8 .
Topoisomerase II (TOP-2) is a promising molecular target for cancer therapy. It is a nuclear enzyme that is essential for cell survival. TOP-2 is involved in various stages of DNA replication as well as chromosome assembly and segregation. It acts by breaking the double-stranded DNA and promoting the relaxation of over-coiled strands 9,10 . Therefore, it is responsible for the topological changes in double-stranded DNA that are required prior to replication and transcription. TOP-2 exerts its function via introducing a protein-bridged cut in DNA strands 11 . A higher level of TOP-2 expression and a prolonged intracellular half-life is observed in malignant cells over normal ones making the enzyme a potential target for anticancer therapy. Drug candidates that target TOP-2 can be classified as TOP-2 poisons and TOP-2 catalytic inhibitors. The TOP-2 poisons exert their function by increasing the covalent binding between the TOP-2 and DNA. This group includes most of the clinically relevant compounds e.g. doxorubicin (DOX). The TOP-2 catalytic inhibitors act by halting the activity of TOP-2 which is crucial for tumour survival 12,13 .
DNA intercalators and TOP-2 poisons usually exhibit three pharmacophoric features; a polyaromatic planar structure (chromophore) to be placed between two neighbouring DNA nucleobases 14 , a cationic centre that interacts with the negatively charged phosphate group on DNA nucleobases, and a groove-binding site which stays in the DNA minor groove 15 . Some reported DNA intercalators and TOP-2 poisons are depicted in Figure 1 [16][17][18] .
Immense research is taking place on the development of novel TOP-2 inhibitors. The journey for the development of a new drug candidate from drug discovery to FDA approval is an economically exhausting process with a low degree of success and uncertainty 19,20 . Repurposing drug candidates that are already approved by the FDA can surpass such time and cost resource consumption; meanwhile, reducing the incidence of developing a new agent with significant side effects that may impact the health of patients 21,22 . Repurposing FDA-approved drugs may also reduce the chemical consumption that is exerted in the development process which is a global quest for climate change. In addition, the utilisation of some in-silico approaches, such as molecular docking and molecular dynamics have a crucial role in the drug repurposing process by affording deep insights about the affinity of the repurposed compounds to the new specified targets and putting eyes on the possible capability of these compounds to be dedicated for new therapeutic uses along with their primary therapeutic uses [23][24][25] .
In this respect, antibiotics are fruitful candidates for drug repurposing. Numerous antibiotics may have protein and nucleic acid targets that exhibit similarities with those of the eukaryotic cells. For example; antibiotics that involve complexing with DNA or the associated proteins such as anthracycline and ionophores. This interaction with biologically relevant macromolecules provokes an antitumor potential of antibiotics. Those classes are either clinically approved or investigated for their antitumor activity [26][27][28][29][30] . In addition to providing a new therapeutic candidate, the repurposing of antibiotics may find a new role for drug molecules that lost their clinical relevance due to the development of bacterial resistance 31 .
Consequently, the rationale of this study is based on screening a reasonable number of available FDA-approved antibiotics that may interact with nucleic acid synthesis or replication. Also, they may share the same previously described pharmacophores commonly retained by most TOP-2 inhibitors. Therefore, 138 antibiotics belonging to aminoglycosides, lincosamides, macrolides, oxazolidinones, peptidyl transferases, streptogramins, sulphonamides, tetracyclines, quinolones, and other classes were in silico tested for their activity against TOP-2 using molecular docking. Then, some of the most promising and commercially available antibiotics were subjected to molecular dynamics simulations for 200 ns to get deep insights regarding their exact stability within the receptor pocket of Top-2. In addition, the MM-GBSA calculations for the frontier members were calculated. Additionally, the antitumor activities of the most promising candidates were investigated against three cancer cell lines (breast, liver, and colorectal) using DOX as a reference drug. Finally, the TOP-2 enzyme inhibition assay for the outstanding promising antibiotics compared to DOX as an FDA-approved TOP-2 inhibitor and DNA intercalator was carried out to confirm the proposed rationale.

Materials and methods
SP, RO, AZ, CL, and ER (as ethyl succinate) were provided from NODQCAR at a purity > 99.87%. Human mammary tumours, hepatocellular carcinoma (HepG2), and colorectal cancer (HCT-116) cell lines were obtained from VACSERA; Egypt. RPMI 1640 doped with 10% FBS was provided from VACSERA; Egypt. The inhibitory potential against TOP-2 was assessed by the Topo GEN assay kit (TOPOGEN, CO, USA).

Docking studies
The molecular docking studies were used to screen the binding interactions of the investigated antibiotics utilising the MOE 2019 suite 32 .
Preparation of the investigated compounds. PerkinElmer ChemOffice Suite 2017 toll was employed for drawing the screened compounds which were then made available for the docking process as previously described in the default procedure 33,34 . By importing them into one database and saving them as an MDB file, the screened compounds and the co-crystallised inhibitor (EVP) were ready for the docking process towards the human topoisomerase II-DNA complex.
Preparation of human topoisomerase II-DNA complex target. Using the protein data bank, the human topoisomerase II-DNA complex X-ray structure was downloaded (PDB code: 3QX3, https://www.rcsb.org/structure/3QX3). The target receptor was protonated, broken bonds were corrected, and the entire protein was energetically minimised to prepare for the molecular docking process as discussed previously in detail 35,36 .
Molecular docking against the prepared target complex. The general docking process was started after the file for the prepared active site was loaded. The docking process site was (Ligand atoms), the scoring methodology was (London dG) and the placement methodology was (triangle matcher). Rigid receptor docking was utilised as the refinement methodology and the scoring methodology was GBVI/WSA dG with the selection of the best 10 poses for each tested compound from 30 different poses. Subsequently, the investigated ligands' MDB file was uploaded to the program and the calculations of the running docking were automatically executed. Following completion, the obtained poses were examined, and the best ones with the highest scores, acceptable RMSD values, and better ligand-enzyme interactions were chosen and saved for further investigation 37,38 .

Molecular dynamics simulations (MDs)
The MD simulations 39,40 for the six studied complexes (SP-3QX3, RO-3QX3, AZ-3QX3, CL-3QX3, ER-3QX3, and EVP-3QX3) were performed using the Desmond package (Schr€ odinger LLC) 41 . Moreover, the thermal_mmgbsa.py python script of Schrodinger was applied to calculate the Molecular Mechanics Generalised Born Surface Area (MM-GBSA) energies 42 for the aforementioned six complexes. The detailed methodologies were depicted in the Supplementary Information (SI 1 and SI 2).

Biological evaluations
The antiproliferative activities of the most promising antibiotics against human breast, liver, and colorectal cancer cell lines To determine the cytotoxic concentration of the tested antibiotics. Antibiotics were transferred into flat-bottomed cell culture microtiter plates (96 well plates, TPP-Swiss) at a concentration of 179.2, 133.5, 241.9, 227.3, and 205.5 mM for SP, RO, AZ, CL, and ER, respectively. An eight-stage four-fold dilution of the antibiotic with the culture media was performed. Cells were seeded (2 Â 10 5 cell/ml; 50 mL) in the wells and incubated for 24 h at 37 C in a humidified and 5% CO 2 atmosphere.
The viability of cells was determined using the tetrazolium-saltbased biochemical reaction, MTT assay 43 , where the cell's treatment medium was decanted and washed thoroughly using a 200 mL buffering system (PBS) twice for 3 min each. This was followed by the addition of MTT (50 mL), and cells were incubated with the dye for another 4 h at 37 C. The dye was discarded, and acidified isopropanol (0.4%, 50 mL) was dispensed to each well to dissolve the formazan crystals. The purple formazan colour was measured at a wavelength of 570 nm and a reference wavelength of 630 nm using an EL-x 800 microplate reader. Results were compared to a positive control (cells and solvent). DOX was used as standard (920 mM). Results were expressed as the concentration of antibiotics that inhibited the growth of cells by using the following equation: Viability % ¼ OD test materials x 100/OD of cell control, then, the IC 50 was calculated.

Topoisomerase II inhibitory assay
The drugs which showed good activity in silico were tested in vitro to estimate their inhibitory potential against TOP-2 using the TopGEN plasmid-based kit according to the user manual protocol TG100110. Results were reported as IC 50 and compared to DOX.

Results and discussion
In silico findings

Molecular docking studies
The molecular docking studies were used to screen the binding interactions of the investigated antibiotics utilising the MOE 2019 suite 32,42,44 . Molecular docking studies were employed to unveil the suggested binding interactions 45,46 of these investigated compounds against the human topoisomerase II-DNA complex (PDB ID: 3QX3), with the native co-crystallised ligand (EVP) as a reference control.
First, the accuracy and suitability of the used force field and the docking protocols were validated via re-docking the co-crystallised EVP inhibitor against the human topoisomerase II-DNA complex target receptor. Consequently, a low RMSD value (1.42 Å) was obtained, enclosing the validity of the Amber10: EHT force field 20,[47][48][49] ; also, a similar binding mode was observed, as depicted in Figure 2. The 2D superimposition of the native and redocked co-crystallised EVP is shown in Figure 2.
Molecular docking was conducted for screening the investigated compounds utilising the co-crystallised ligand (EVP) as a reference standard. Hence, deep insights into the binding interactions of the screened compounds against the DNA-topoisomerase II complex could be attained. The binding scores of the screened antibiotics towards the human topoisomerase II-DNA complex (PDB ID: 3QX3) compared to the co-crystallised inhibitor (EVP) are represented in the Supplementary Data (Table SI 1).
The binding scores, RMSD values, as well as the details of the binding interactions with their corresponding bond types for the most promising and commercially available five compounds (spiramycin (SP), roxithromycin (RO), azithromycin (AZ), clarithromycin (CL), and erythromycin (ER)), besides, the co-crystallised ligand (EVP), are depicted in Table 1.
Analysing the binding interactions of the re-docked native inhibitor (EVP) revealed that EVP displayed a binding score of À10.51 kcal/mol with an RMSD value of 0.93 Å. Furthermore, it is worth noting that the dioxane nucleus of EVP could form Hbonds with the key amino acids GLN778 and MET782 at distances of 2.94 and 3.72 Å, respectively. Besides, the pyran nucleus of EVP could interact with key nucleobases forming an H-bond with DG13 and an H-pi bond with DA12 at distances of 3.37 and 3.75 Å, respectively. Finally, the terminal -OH of the cyclohexyl moiety of EVP could form H-bond with the key amino acid ASP479 at a distance of 2.70 Å, as shown in Figure 3.
Compounds AZ, CL, and ER showed a docking score of À11.72, À10.99, and À10.83 kcal/mol, respectively, which is higher than that attained by the co-crystallised inhibitor.
Compound AZ could compose H-bonds with the key amino acids ASP479, GLU477, and ARG503 at distances 2.93, 3.07, and 3.29 Å, respectively. Additionally, it could interact with the key nucleobase DT9 forming H-bond at a distance of 2.95 Å, as shown in Figure 4(A,B).
Compound CL, on the other hand, could compose H-bonds with the amino acids ASP479 and LYS505 at distances of 2.86 and 3.31 Å, respectively. In addition, it could interact with the key nucleobase DG13 forming two H-pi bonds at distances of 3.87 and 4.25 Å, as shown in Figure 4(C,D). Finally, compound ER could compose H-bond with the key amino acid ASP479 at a distance of 3.33 Å. Moreover, it could interact with the nucleobases DG10 and DG13 forming one H-bond (3.01 Å) and one H-pi bond (4.57 Å), as shown in Figure 4(E,F).
Furthermore, compounds RO and SP exhibited the best binding scores among selected compounds. Hence, compounds RO and SP revealed binding scores of À11.97 and À12.07 kcal/mol, which is superior to that of the co-crystallised inhibitor. Compound RO could compose H-bonds with the two key amino acids ASP479 and ARG503 and the amino acid LYS505 at distances 2.87, 3.32, and 3.11 Å, respectively. In addition, compound RO could interact with the nucleobase DT15 forming H-bond at a distance of 2.99 Å, as shown in Figure 5(A,B). On the other hand, compound SP could compose H-bonds with the key amino acid ASP479 at a distance of 2.71 Å and the key nucleobase DG13 at a distance of 3.03 Å, as shown in Figure 5(C,D).

Molecular dynamics simulations (MDs)
MD simulations were performed for 200 ns to monitor the exact behaviour of the superior and commercially available antibiotic members (SP, RO, AZ, CL, and ER) within the binding pocket of the topoisomerase II-DNA complex. This was done to clarify and deeply understand the protein-ligand interactions of the studied compounds 37,51,52 . Moreover, the EVP co-crystallised inhibitor was used as a reference.
RMSD analysis. In order to investigate the degree of deviation for the studied complexes regarding the initial position of their heavy atoms in a quantitative manner, the RMSD of the protein Ca  atoms was recorded during the 200 ns of the simulation. It is an important parameter to judge the stability within the protein conformational changes all over the simulation time.
The protein RMSD values of the six studied complexes are described in Figure 6. The protein RMSD for SP showed the most stable behaviour during the simulation time with an RMSD value of less than 3.00 Å. On the other hand, EVP, CL, and ER RMSD values were less than 4.00 Å indicating stable unchanged conformations for both proteins of CL and ER compared to that of the cocrystallised (EVP) inhibitor. Finally, the RMSD for RO and AZ proteins deviated at 60 and 40 ns, respectively, to 5.00 Å indicating slight conformational changes of the target TOP-2 protein.   Next, the RMSD of the frontier ligands concerning their initial position inside the active site of the TOP-2 was reported in Figure  7. Among other compounds, AZ was the most stable inside the active site; it held its position with RMDS of 1.00 Å from the beginning of the simulation till around 150 ns, where it moved by 2.00 with respect to its recent position and held it towards the end of the simulation. The co-crystallised ligand (EVP) and compound RO showed similar behaviours. The RMSD of both compounds was around 3.00 Å concerning their original site. Compound SP moved by 5.00 Å concerning its initial position, while compound ER moved by 6.50 Å, where both stayed steady towards the end of the simulation. Finally, compound CL showed the highest RMSD, and could not hold a fixed position; the compounds moved by almost 7.00 Å concerning its initial position. Snapshots of the ligands-proteins during a different simulation time are presented in Figures SI 1-SI 6 (SI). Figure SI 7 (SI) reports the RMSF (root mean square fluctuation). Where, most proteins fluctuated within the range of 3-4 Å, with Cand Nterminals fluctuating at around 8 Å. Such change is acceptable within such a large system.
Finally, regarding the histogram of the EVP-3QX3 complex (Figure 8(F)), ASP479 was noted to be the main amino acid included in the interactions with the co-crystallised EVP inhibitor (125%). It formed mainly H-bonds (115%) and a few water bridges (10%) to EVP. Also, the contributions of SER480 reached 80% through the formation of water bridges only to EVP.
Therefore, only SP (195%) and RO (140%) members showed a closely similar binding interaction to ASP479 amino acid of the binding pocket of the topoisomerase II-DNA complex compared to that of the co-crystallised EVP inhibitor (125%).
Heat maps analysis. The heat map analysis was considered to monitor the total number of interactions for each amino acid residue regarding the simulation time (200 ns). The heat maps for the six studied complexes (SP-3QX3, RO-3QX3, AZ-3QX3, CL-3QX3, ER-3QX3, and EVP-3QX3) are represented in Figures 9(A,B) and SI 8-SI 11, respectively.
The heat map of the SP-3QX3 complex (Figure 9(A)) showed that ASP479 could form more than one type of interaction with SP all over the simulation time (200 ns). It showed the most apparent interactions from (0-10 and 20-130 ns) of the simulation time.
On the other hand, the RO-3QX3 heat map (Figure 9(B)) clarified that ASP559 interactions with RO were of high intensity all over the time of the simulation (200 ns). This was followed by Asp479 interactions, divided by the simulation time (200 ns), showing the most intense interactions from 10 to 30 ns. Besides, the AZ-3QX3 heat map ( Figure SI 8) showed that GLU447 amino acid interactions with AZ were more intense at (40-110 and 120-200 ns) of the simulation time. However, the HIS775 interactions were clearer from 50 to 190 ns of the simulation time.
However, the heat map of the CL-3QX3 complex ( Figure SI  Finally, the EVP-3QX3 heat map ( Figure SI 11) clarified that the interactions of ASP479 to the co-crystallised EVP inhibitor were of high intensity from the start to the end of the simulation time (200 ns). Besides, SER480 contributions were mostly from 35 to 190 ns of the simulation time.

MM-GBSA calculations
The average MM-GBSA binding energy was calculated using the thermal_mmgbsa.py python script of Schrodinger 41,53 to calculate Van der Waals, Hydrogen-bonding, Generalised Born electrostatic solvation, Coulomb, Lipophilic, and Covalent binding energies. The calculated energies for compounds SP, RO, AZ, CL, ER, and EVP at the topoisomerase II-DNA complex are represented in Table 2. Based on the results in Table 2, we can observe that the DG Binding (À73.77 kcal/mol) and VdW (À74.17 kcal/mol) energies of ER were superior to other members including the co-crystallised EVP inhibitor. Also, the Coulomb (À184.81 kcal/mol) and Solv_GB (199.54 kcal/mol) energies for AZ were higher than those of the studied candidates. The Lipo energy of AZ (À15.16 kcal/mol) was very close to that of EVP (À17.81 kcal/mol). On the other hand, SP showed the highest Covalent energy (3.26 kcal/mol) within the binding pocket of the topoisomerase II-DNA complex. However, RO formed the best H-bond energy (À2.08 kcal/mol) towards the binding pocket of the topoisomerase II-DNA complex compared to the other examined drugs.

Biological evaluations
The antiproliferative activities of the most promising antibiotics against human breast, liver, and colorectal cancer cell lines The cytotoxicity (IC 50 ) assay was tested against three cancer cell lines namely breast cancer cell line (MCF-7), hepatocellular carcinoma (HepG-2), and human colorectal carcinoma (HCT-116) utilising the MTT assay. In addition, the IC 50 was compared to that of DOX as a reference drug. Results showed a comparable IC 50 to that attained by DOX ( Figure 10).

Topoisomerase II inhibitory activity
The potential of the tested drugs to inhibit the ability of TOP-2 to decatenate an intertangled mass of DNA was assessed. In addition, the selected macrolides were further investigated to determine their inhibitory potential on the TOP-2 enzyme using DOX (an anthracycline antibiotic with potent inhibition towards TOP-2) as a positive control. The acquired results agreed with the cytotoxicity assay, and the obtained data indicated that the tested antibiotics have a promising inhibitory potential against the TOP-2 enzyme relative to DOX.
The recorded IC 50 for DOX (positive control) was 6.37 ± 0.38 mM. In addition, potent TOP-2 inhibitory activity was recorded for ER and RO with IC 50 of 11.58 ± 0.70 and 13.07 ± 0.77 mM, respectively. On the other hand, AZ showed moderate activity with an IC 50 of 36.09 ± 2.11 mM. Finally, CL and SP exhibited moderate IC 50 of 25.58 ± 1.08 and 21.23 ± 1.19 mM, respectively ( Figure 11). The order of IC 50 of different antibiotics was slightly different in cytotoxicity and enzyme assay, especially for ER. Variations may explain the tested antibiotics' cellular/nuclear permeability in different cell lines.

Structure-activity relationship (SAR)
Based on the molecular docking binding scores for the tested antibiotics (Table SI 1), a SAR study was conducted opening eyes to promising anticancer pharmacophores encountered by the screened antibiotics. Interestingly, the following outcomes were attained ( Figure 12): Among the screened antibiotics, it was disclosed that macrolides and streptogramins exhibited the most promising binding scores and, thus, higher anticancer potential. Moreover, among screened aminoglycosides, it was revealed that plazomicin displayed the best binding affinity to TOP-2, whereas, among screened lincosamides, it was unravelled that lincomycin showed the best binding score. Besides, regarding screened macrolides, it was shown that all compounds experienced promising binding scores that are better than that attained by the co-crystallised ligand assuring their stability at the target receptor. In particular, it was disclosed that tilmicosin showed the best binding score among the investigated macrolides. Furthermore, considering screened oxazolidinone, it was disclosed that radezolid showed the best binding affinity to the target receptor, whereas, among screened peptidyltransferase, it was revealed that valnemulin has the best docking score. In addition, regarding screened streptogramins, it was shown that linopristin, pristinamycin IA, quinupristin, and virginiamycin S1 antibiotics exhibited promising docking scores that are better than that attained by the co-crystallised ligand. In particular, it was disclosed that linopristin showed the best docking score among the screened streptogramins. Additionally, regarding sulphonamides, tetracyclines, and quinolones, it was unveiled that the compounds with the best binding score in their corresponding class were terephtyl, lymecycline, and prulifloxacin, respectively. It is worth noting that, among screened antibiotics, sulphonamides and quinolones exhibited the lowest binding scores, and thus, less anticancer potential could be attained.

Conclusions
This research aims to identify a potential TOP-2 inhibitor that is considered a promising molecular target for cancer therapy. Herein, the molecular docking studies of 139 antibiotics belonging to different classes revealed that SP, RO, AZ, CL, and ER were among the most promising and commercially available members. Afterward, the MD simulations for 200 ns clarified that AZ was the most stable inside the active site of the topoisomerase II-DNA complex, followed by the co-crystallised ligand (EVP) and RO, which showed similar behaviours according to the RMSD analysis. Also, the histogram analysis showed that only SP and RO members showed a closely similar binding interaction to ASP479 crucial amino acid of the TOP-2 receptor, compared to that of the co-crystallised EVP inhibitor. Moreover, based on the MM-GBSA calculations, it was observed that the DG Binding energy of ER (-73.77 kcal/mol) was superior to the other MD-studied compounds, including the co-crystallised EVP inhibitor. On the other hand, the MTT assay against MCF-7, HepG-2, and HCT-116 cancer cell lines clarified that SP and CL showed promising anticancer potentials on the MCF-7 cell line with IC 50 values of 0.67 ± 0.43 and 1.39 ± 0.15 mM, respectively, compared to that of DOX (0.49 ± 0.1 mM). Besides, AZ and CL exhibited good anticancer potentials against the HCT-116 cell line (2.08 ± 0.05 and 2.74 ± 0.42 mM, respectively) compared to 1.6 ± 0.05 mM for DOX. Furthermore, the TOP-2 enzyme inhibition assay was carried out to confirm the proposed rationale compared to DOX as a TOP-2 inhibitor and DNA intercalator. Notably, potent TOP-2 inhibitory potentials were recorded for ER and RO with IC 50 values of 11.58 ± 0.51 and 13.07 ± 0.65 mM, respectively, compared to DOX (6.37 ± 0.21 mM). Additionally, the SAR study revealed that macrolides and streptogramins exhibited the most promising binding scores among the tested antibiotics and thus expected better anticancer potentials. Finally, it is recommended to evaluate all the superior antibiotics (either examined or not) using more advanced preclinical and clinical studies to get a more accurate conclusion regarding their antitumor activities.